Modern implantable pacemaker systems (comprising an implantable pacer having leads that connect the pacer to the heart) include the ability to sense the IEGM, convert it to electrical (data) signals, and telemeter the IEGM signals to an external device (typically referred to as a "programmer"). At the external device, or programmer, the IEGM signals are processed to recreate the IEGM and display and/or print it for analysis, or store it for subsequent analysis. The IEGM thus displayed/printed or stored is comparable with, and in many ways superior to, the conventional electrocardiogram (EKG), sensed through skin electrodes.
The IEGM obtained via telemetry from an implanted pacemaker has heretofore been useful primarily to understand the relative timing between the various cardiac/pacing events manifest by it, and for comparison of relative amplitudes between various portions of the IEGM. In a dual-chamber pacemaker, i.e., one that can sense in both the atrium and the ventricle, the IEGM readily indicates the occurrence of, and timing between, either a natural or paced atrial depolarization, and either a natural or paced ventricular depolarization. The timing of such events is of critical importance to the cardiologist or physician who is analyzing the performance of the implanted pacemaker.
Unfortunately, heretofore, there has been no way to calibrate the magnitude (amplitude) of the displayed IEGM signal. That is, unlike conventional skin electrocardiographic (EKG) devices that use skin electrodes to sense cardiac activity, and which conveniently provide a "calibrate signal" that allows the physician to calibrate and verify the absolute volts per division that appear on the paper or screen trace of the EKG device, there is no such calibrate feature provided by IEGM display systems. This is because in an IEGM display system, the electrogram signals displayed originate within or on the myocardiogram of the heart, and are sensed through the pacemaker leads and appropriate sensing circuits. Because the sensed IEGM originates inside of the body, there has not heretofore been a convenient mechanism for noninvasively comparing such sensed signals to a known standard. Further, once sensed, the IEGM signals are then encoded by the implanted pacemaker, telemetrically transferred to the external programmer, and then decoded, digitized, and formatted for display on the programmer screen by the external programmer. All of this signal telemetering and processing may introduce significant variations in the amplitude of the signals thus displayed. As a result, any "calibrate" signal inserted into the display at the programmer would not take into account scale factor variations occurring in the pacemaker or in the telemetry process. As a result, the physician or cardiologist is not able to verify the "gain" of the pacemaker-programmer system from one patient visit to another, or of one patient to another (where comparative studies are desired).
The "gain" of the pacemaker-programmer system represents how much the electrical signals measured on the myocardium are amplified or attenuated as such signals are sensed by the pacemaker's sensing circuits, encoded by the implanted pacemaker, telemetrically transferred to the external programmer, and then decoded, and formatted for display on the programmer screen by the external programmer. Knowing such "gain" could be an important and useful factor when tracking the performance of the pacemaker-programmer system for a given patient, or for a group of patients. A significant change in such "gain", for example, could signal a condition that needs to be investigated either with the patient and/or the pacemaker/programmer equipment used to monitor the patient. Little or no change in such "gain" could signal that the monitoring conditions are essentially the same as existed on a prior occasion.
In order to calibrate the "gain" of the pacemaker/programmer system, it is necessary to insert a signal of known amplitude (e.g., a calibrated reference voltage) into the data stream that is being telemetered from the pacemaker to the programmer. While some prior art pacemakers have deliberately inserted a marker signal into the telemetered data in order to signal the occurrence of certain sensed events, see, e.g., U.S. Pat. No. 4,374,382, no known pacemaker generates and then selectively inserts (e.g., upon receipt of an externally-generated calibrate command signal from the programmer) a calibrated reference voltage into the telemetered data for the purpose of calibrating the amplitude of the IEGM or other data telemetered from the pacemaker.
What is needed, therefore, is a system that selectively inserts a known reference signal into the telemetry data stream in order to allow the pacemaker/programmer "gain" to be verified. The present invention advantageously addresses this and other needs.